Electromagnetic device



Oct 13, A. L. THURAS ELECTROMAGNETIQ DEVICE Filed 0619.8, 1923 iPatented Oct. 13, 1925.7

- UNITED STA.

TES

ALBERT L. THURAS, OF EAST ORANGE, NEW JERSEY, ASSIGNOR T WESTERNIXLIEIC- TRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y,A CORPORATION OFNEW' YORK. v

Ermcnomaennrrc DEVICE.

Applicationled October 8, 1923. Serial No. 667,154.

T o all whom it may concern.:

Be it lrnownvthat I, ALBERT L. THURAS, a citizen of the United States,residing at East Orange, in the county of Essex, State of New` Jersey,have invented certain new and useful ,Improvements yin ElectromagneticDevices, of which the following is a full. clear, concise, and exactdescription.

This invention relates to electromagnetic devices and more particularlyto magnetic structures for telephone receivers and the like.

The object of the invention in general is to provide a vibratory systemparticularly adapted for telephoneapparatus which, as a whole. will havesuch variable mechanical properties with frequency and displacement thatthe vibrations of the system will only be limited by its internalmechanical resistances and the radiation resistance of the air, withinthe range of the frequencies ofim- -portance in sound reproduction.

In accordance with the general features of the invention the vibratorysystem of a magnetic device is provided with elastic means7 which atzero frequency, provides a Y force opposing the magnetic pull on thearmature in a manner such as will give a resultant restoring iforceproportional to the displacement of the armature, and which,

under operating conditions makes the vibratory system resonant atsubstantially`\ all frequencies ofA importance in sound reproduction.

In its preferred embodiment, the invent-ion provides in anelectromagnetic device responsive to sound wave energy vibrations,

an armature operable in response to said vibrations, a vlbratorv memberassociated with the armature an a plurality of thin sheets of materialsuch as paper or aluminum foil supported in the form of a loose packagainst each side of the vibratory member, the sheets in each vpackbeing sufficiently loose to provide air films between adjacent sheets.

When the armature of a telephone ro ceiver of the balanced armature typeis in its neutral position in the magnetic field,- there is` no tendencyfor it to move toward the pole pieces. However, any displacement fromits neutral position will create a pull which increases very rapidlywith the displacement and tends to draw the armature to the polepieces.v Heretofore, in response to vibrations at the various ceiversemploying balanced armatures,' in order to prevent the armature fromstriking the pole pieces it has been necessary to rely upon eitherthestiffness of the diaphragm` alone or the stiffness of the diap hragm andSome auxiliary means such as a spring att-ached to' the armature. Thesemeans were necessarily designed to provide an elasticity which wassufficient to overcome the pull on the armature for the o5 maxlmumdisplacement, no 'particular attensystem in which the amplitudes of-vibration were not proportional to the forces applied thereby causingdistortion and the large elasticity with its larger mass resulted in aninefficient system for radiating sound) energy. In-an ideal magneticvsystem, the

pull on the armature would be inversely proportional to the square ofthe separation between the armature'and the pole pieces. The pull vs.displacementcharacteristic in this-case would be represented by aparabola.

This is, however, only roughly true in the commercial types of receiversbut the displacement characteristic, due to the magnetic field, is ofthis general form. It is,

therefore, necessary in order that no distortions will result due to theunbalance of the elasticity in the magneticeld to provide a restoringelasticity having the negative characteristic of the elasticity of themagnetic field. If these opposing elasticities were exactly balancedthearmature would not return to its' neutral position when displaced. Itis, therefo e, desirable, in order to insure a large and accurateresponse and prevent freezing of the armature that the mechanicalelasticity be slightly greater by a constant amount for all positions ofthe armature than the magnetic elasticity. This will provide a restoringforce proportional to the di lacement of the armature.

e above discussion refers only to the static, or non-operatingconditions of the vibrating system but it is also necessary to considerthe system when operatin in refrequen- Y cies`- within the sound range.In order to provide a system which'is reasonably responsive to all ofthe frequencies of the impressed.,`

mechanical constants of thin air films, the

- .spaced relation.

,coils and secured to the pole pieces by means elasticity of which as iswell known varies,

within a certain frequency range, substan-A tially as the square of thefrequency. Reference is had to E. C. Ventes paper published in thePhysical Review, vol. 10, page 39, and I. B. Crandalls paper publishedin the Physical Review, vol. XI, page 449.

Referring to the drawings, Fig. l-shows a magnetic system suitable fortelephone receivers 'employing the invention in one form;

Fig. 2 is an end view of the magnetic system shown in Fig. l;

Fig. 3 shows a modification of the invention;

Fig. 4 is a cross sectional -view of the modification shown in Fig. 3;

Fig. 5 is a modified form of the invention as shown in Figs. 3 and 4;and

6 shows displacement vs. elasticity characteristic curves forvstructurcsprovidacting upon each otherwithout considering ed with and without theinvention.

Y In the construction shown in Figs. 1 and 2, the magnetic structurecomprises an electromagnet 7 to the opposite ends of which magnetic polepieces 8 and 9 fare respectively secured. Each of the pole pieces isprovid- :ed with a pairof pole extensions 10, 10 1 which are separatedto receive a pair of translating coils 11, 11 therebetween and inExtending betweenthe yof screws 12, 12are torsion members 13, 13carrying armature 14firmly secured to the members by brazing or othersuitable means. The pole piece 8 is secured to the upper side of one endof the magnet 7 and the pole piecei9 tothe lower side lof the other endso as to bring the pole extensions 10, 10

of one pole piece in overlapping relation to those of the other polepiece. The coils 11, 11 are located on the opposite side of the members13, 13 with the ends 15 and 16 of the armature extending through theopenings in the coils, the openings being made of such a size as topermit free movement of `the armature during operation of the receiver.The ends and 16 of the varmature 14 extend between the opposing pairs ofthe pole extensions 10, 10 of the pole pieces 8 and 9 and the end 15carries a rod 17 secured to the vibratory diaphragm 18. Also secured tothe end 15 of the armature 14 is a vibratory member 19 parallel to thediaphragm 18. l'he ends of the vibratory member 19 may be supportedbetween the elastic cushions 20 at each end by means of members 21 and22 at one end and members 23 and 24 at the other. The cushions 20 arebuilt up of a number of sheet-'fs of thin,niatei11alsuchas, aluminumfoil or a er an el ual number*V of sheets beinfT disposed on each sideof the vibratory member 19. The ends of the vibratory member 19 andalso' the ends ofl the vsheets are"pejrfo rated to lreceive the screws25, 25.' `The members 21, 22, 23 and 24 are beveled on the surfacesbetween which the cushions 2O are clamped so `as to provide a largerspace for ness with frequency may be of'a more coniplex function due toa grcaterLor lesser number of layers being in motion at differentfrequencies. In order to compensate for the magnetic pull on thearmature due to the steady field, it is only necessary to take into yaccount the elasticity providedby the sheets the elasticity due to theair'- cushion.1 However, in order to cause the vibratory system torespond withthe greatest amplitude for-r all of the impressedfrequencies, advantage is taken of the elasticity Aof thev air betweenadjacent leaves of the cushions, It is well known that if a plate isplaced inrclose face to face relation withv a vibrating diaphragmtheelasticity of the air will increase substantially as the square of thefrequency. For this effect the air film should have a ythicknessapproximately of the orderl of A .0005 of an inch. Inasmuchy as thedisplace-` ment of thevibratory member of loud speaking receivers,electromagneticrecorders and reproducers is considerably larger thanthis, it is necessary, in order to obtain the same variation inelasticity with respect` to the frequency toprovide alpliirality ofspaces each of which may besomewhat smaller but which will totalanlamount sufficientto permit the vibratory member in such devices Etorgspond properlyl to thev largest lamounts of energy which may be used.I

In the modification shown in Figs. 3 and-1 there is provided adplu'nger31 attached to the ends of the armature by means of. a rod 30 andoperating in a cylindrical chamber 32 attached to a plate 33 which inturn is secured to the magnet 7. Disposed on one sdeof the plunger 3lare al plurality of` thin discs 34 of aluminum foil or paper and on theopposite side of the vplunger are a. plurality of similar discs 35:perforated at their centers to receive the iod 30. The operation of thismodification is substantially the same as that disclosed in Figs. l and2. Fig. 5

. This arrangement discloses a further modification of the structureshown in Figs. 3 and 4. Inthis modiication, heavier discs 36 and 37 ofdifferent thicknesses -are used between the aluminum discs 34 and 35respectively which results in a variation of the effective mass as wellas the elasticity with frequency.

Referring to the curves shown inFig. 6, curve A represents the variationin elasticity with displacement due to` the magnetic pull on thearmature. Curve B represents the variation in the mechanical elasticitywith' the displacementas obtained by the cushioning means in accordancewith the inventiolr., Curve C represents the variation in elasticitywith displacement of the armature in the same receiver without thecushioning device of the invention but with an ordinary spring toovercome the force of the magnetic field. Curve D represents theresultant elasticity due to the combined effec-ts of the elasticity ofthe magnetic field shown by curve A and the mechanical elasticity of thecushioning means shown by curve B. Curve D is obtained by subtractingcurve A- from curve B. It will be noted from this curve -that theelasticity decreases as the displacement of the armature increases. Thlsre- ,sults in distorting the electrical sound energy which is impressedon the receiver.

A satisfactory cushioning device may be designed by using one hundredcircular layers of .6 millpaper with an area of approximately 1/2 squareinch, where the separation is such that the fraction of the totalspaceoccupied by the pa er is approximately .2.

as been lfound to be effective for a receiver of the type disclosed inPatent 1,365,898 of January 18, 19211 to H. C. Egerton. The number ofleaves, the dimensions and the space factor will' obviously vary 'formagnetic systems of different construction. n

Although the invention has been shown 'as applied to electromagneticsystems for telep one receivers it may be readily adapted to anyelectromagnetic device having a vibratin armature.

e invention claimed is.:

1. In a magnetic structure, a magnet producing a steady magnetic field,an armature disposed in said ield, and cushioning means exerting a forceon said armature which varies at zero frequency with displacementof thearmature in the same lmanner as the force due to the magnetic pull onthe armature.

2. In a magnetic structure, an armature,

I a magnet producing a force on said armature varying 1n a predeterminedmanner with the displacement of said armature, and cushioning meansproducing an opposing force on' said armature varying with thedisplacement in the same manner as the magnetic force. 3. In a magnetlcstructure, a magnet, a

pair ofpole pieces carried by said magnet, avibratory armature inoperative relation with said pole pieces, a member secured to saidarmature and adapted to vibrate therewith, and cushioning means forapplying restoring forces through said member to said armature saidforces varying with the displacement in accordance with the magneticforce due to said magnet.

4. In a magnetic structure, the combination with a magnet producing asteady magnetic field and pole pieces carried by said magnet, of avibratory system comprising an armature is operative relation to saidwpole pieces, a vibratory member attached to said armature, and aircushioning means applied to said member for `maintaining the elasticityof said system substantially constant for all positions of the armaturein the magnetic field.

5; A magnetic structure comprising a magnet producing a steady magneticeld, pole pieces attached to said magnet, an armature in operativerelation to said pole pieces, and means for compensating forthe magneticpull on said armature, said means comprising a plurality of air cushionsdise posed on each side of said member whereby a constantresultantelasticity is provided for all displacements of the armature.

6. A magnetic structure comprising a magnet producing a `steady magneticfield, pole pieces attached tosaid magnet,'an armature in operativerelation to said pole pieces, and means for compensating for themagnetic pull on said armature, said means comprising a vibratorymember, a plurality of thin sheets of material, and means for supportingsaid sheets loosely on each side of said vibratory member wherebycushions are formed to provide an elasticity greater by a constantamount for all displacements of the armature than the elasticity due tothe magnetic field.

7. In an electromagnetic device having a steady Jmagnetic field, avibratory system comprising an armature disposed in said field, a plateattached to said armature, a plurality of thin sheets of material, meansfor supporting said sheets on each side of said ',plate, and means forsupporting said sheets loosely in contact with each other and with saidplate whereby cushions are formed providing a restoring forceproportional to the displacement of the armature.

8. In apparatus for' the t-ransmissionof sound wave ener a cushionedvibratory system resonant orall frequencies within the easily audiblerange and having a constant elasticity for all displacements thereof atzero frequency. l

9. In apparatus for the transmission'of soundwave energy a vibratorysystem oomrising a plurality of air cushions provid-y mg a rconstantelasticity for all displacements at zero frequency and;anne-elasticityVv variable as the square ofthe vfrequencies ofthevibrations Within the range 1of importance.

10. Ajvibratory systemfor the transmission of soundwave energy, saidsystemhaving mass and Eelasticity one of Which varies so thatitstotalelastic reactance 4is substantially equal to its totaleective massreactance at all frequencies Within the range of.

tially asthe'square of the frequency, said elasticity being so relatedto the mass that the system as a Whole is resonant 4throughout the soundfrequency range.

12. In a vibratory system for the transmission'of sound energy having aplurality of connected masses in `,which the aggregatedynamiclel'asticity and the aggregate efy.fective mass changeWithfrequency so as to produce resonance throughout the sound frequencyrange.4 v'

13. A vibratory system for the transmission of sound -Wave `energycomprising a pluralityl of connected masses, the aggre ateeffective/mass of which remainsconltant With frequency; andastructureconsisting of a 'pluralityof separate sheets of thin materialand intervening thin 'air films arranged to come into actionsuccesslvely yto produce an elasticity which increasesy substantially astheA square of the frequency tance is just'equal to the aggregateeffective Ymass reactance.y

15. A device iny which thev total effectivev vibrating mass decreases asthe frequency ,quency to be transmitted the elastic reacincreases andthe elasticity increases 'as the being substantially equal: to the mass`reacvfrequency increases, the elastic reactance 1 tance throughout thevoice frequency range'. 16. In a vibratory system for* the transmissionof sound Wave energy, a vibrating ysystem having atotali-resultantconstant elasticity Withl frequency and lhaving a plu--rality of elements Whose aggregate eectivef vibrating mass decreasesas'the square ofl the frequency, to causeV saidsystem to be resonantthroughout` the range.

1923. f f i ALBERT L. THURAS.

Isound frequency 'l l In witness whereof, lI hereunto subscribe myname-this 27th day of September A. D.

